If the people of Scotland and Wales believe it will be the best way to deal with their current situation then they should have a good chance at succeeding with it and so therefore could be considered a good idea. The people may do well under a devolution and should think about it.
The union is stronger with a stronger central government. If power is constantly devolved who sets the limit? Of course these new 'parliaments' and 'assemblies' want more power, they get a thirst for it and believe it is there entitlement for more. One captain sails a ship, not numerous captains- otherwise the ship will crash. The general public has no clue who has what power anymore, and the process is so expensive and downright excessive. It weakens the union in that it sparks the flame of regional nationalism, and then leads to scenarios such as the Scottish independence referendum. We need a strong United Kingdom, with strong leaders that WE elect. Not multiple leaders with tiny fractions of power, it is rather ridiculous. I do emphasise with Wales, NI and Scotland as their small populations means they have far fewer MPs than England. But is that not fair? Is it not fair that England with 53+ million people should have their fair proportion of MPs. I feel as though Scotland, Wales and NI have had more of a say in the UK than they think, you just have to look at recent prime ministers and cabinet ministers.
No, devolution in Scotland and Wales is not a good idea, because the United Kingdom has a strong central government and the strong government aids Scotland and Wales. It would cause confusion and would be more costly to govern Scotland and Wales in smaller units. In addition, national governmnet on a large scale aids in national unity and peace within the region.